A lookup data in a SharePoint can be treated as a choice options that has been maintained in a different List. Hence if one of your field in your List is a lookup then, its value should be the ID[integer] the actual Lookup-Item (i.e., the list item id of the List as a LookUp).
For ex, in a blog site, List, Category has been used as lookup field for the List Post. By default, in SP2013, you’ll get three items for Categories. Say, their Ids are 1,2, & 3.
ClientContext ctx = new ClientContext(targetURL); ctx.Credentials = new SharePointOnlineCredentials(userName, passWord); this.ctx.Load(this.ctx.Web); SP.ListCollection listCol = web.Lists; ctx.Load(listCol); SP.List list = listCollection.GetByTitle("ListTitle"); SP.CamlQuery camlQuery = new SP.CamlQuery(); camlQuery.ViewXml = ""; SP.ListItemCollection listItemCollection = list.GetItems(camlQuery); this.ctx.Load(listItemCollection); this.ctx.ExecuteQuery(); SP.ListItem listItem = listItemCollection.FirstOrDefault(t => t.DisplayName == "NameOfTheListItem"); this.ctx.Load(listItem.FieldValuesAsText); this.ctx.ExecuteQuery(); //[Say, for one of the Id mentioned above.] listItem.FieldValuesAsText["PostCategory"] = 2; listItem.Update(); this.ctx.ExecuteQuery();
To get the id of the listItem Ideas, you’ll have to re-connect to the SharePoint server and get the ID for the text Ideas from the List Category.
NOTE: Here, I have faced a strange issue. If I update any value of the Post Item before updating the lookupField value then the previous made changes were getting overwritten. So I have to ensure that the lookupField value should always get updated first before any other field.